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There are a number of different light bulb styles. One particular style is sometimes called a xe2x80x9ccandelabraxe2x80x9d style and includes those standard type light bulbs as the C7, C9, and T50. For convenience, xe2x80x9cC7/C9xe2x80x9d will be used to designate all candelabra-style lights. The C7 and C9 types they are probably the most popular types of candelabra-style lights for use in holiday lighting, night lights, and decorative lighting systems for homes, restaurants, outdoor events, theme parks, holiday lighting of parks, towns, cities, etc. These bulbs are the small- and medium-sized screw type, threaded bulbs.
The outdoor use of C7/C9 lighting for holiday events, decoration and extravaganzas has steadily increased every year since the mid 80""s. The popularity of the larger C7/C9 lamps was very high in the 50""s, 60""s and 70""s when these lamps were used on the interior trees, exterior trees, bushes, and on the home exterior, until the price of electricity suddenly and rapidly increased, making everyone aware of power consumption. As a result, outside holiday decorations declined to almost nothing for years. The larger C7/C9 style, in particular, fell out of favor due to its high power consumption. Gradually, however, more and more holiday decorators started using the more energy efficient miniature light sets, which became very popular since the early 80""s because they used 30+% less power.
During the mid 80""s, special, for profit, seasonal lighting displays started appearing everywhere. The lack of exterior home decorating created a very profitable business as families wanted to go see lighting displays, once free on homes, now available for a charge in special parks. These parks began using a mix of C7/C9 and miniature light sets and as the parks grew, more and more of them started changing over to the C7/C9 as a primary light source due to the fact that the C7/C9 sets operated electrically in parallel and therefore failed less often compared to the miniature light sets which operated electrically in series. A series lamp failure could mean that half of the lights on a display could go out at one time from the failure of just a single lamp, whereas, the failed lamp in a parallel system would not have this effect on the remaining lamps. The series approach also made trouble-shooting the failed light displays a very difficult task. As most of the miniature lamps are wrapped around metal frames to hold the display""s shape, trouble shooting xe2x80x9csnifferxe2x80x9d tools developed to find the burned out lamp in a series set were prevented from working properly.
As these light display companies grew larger and larger, the requirement for dependability also grew, so more and more of the displays have been converted over to the C7/C9 sets. The popularity of the C7/C9 sets in the displays has now effected the designing public, so more and more homes are again using the C7/C9 style light sets, especially outdoors as these sets are much stronger and more durable, however, the C7/C9 sets do have their own unique set of problems and dangers. Firstly, the C7/C9 sets operate electrically in parallel and are powered by 120VAC. C7/C9 light sets are fused at 5 and 7 amps when using energy efficient lamps. If a lamp is missing, however, then the lamp socket is open, exposing 120V AC at 5-7 amps. This amperage is easily enough to kill a grown adult. The socket is also large enough for a child to insert a finger into it or for a small bush limb to work its way into the socket. In either of these cases the outcome of such a contact be dangerous and possibly fatal; also, they can cause an electrocution or a fire as a to a limb.
In one of these sockets, there is a two-part center (AC Line) terminal. The upper portion of this terminal (namely, the part that contacts the lamp) is a special copper alloy and is slightly bent upwards with one end bent downward towards the bottom of the socket to form a switch contact. Under the downward-bent part is the second part of this two-part terminal. The second terminal is made of brass and pierces the electrical wire under it. When a lamp is screwed into the socket, the center contact of the lamp engages the center terminal of the socket, compressing the downward-bent part of the terminal until it comes into contact with the energized second part of the terminal under it. Then the lamplights. However, this approach has several problems, and that often cause the lamps to fail to light.
Secondly, in order to reduce the manufacturing cost of the new C7/C9 sets, manufacturers are using molded plastic sockets rather than a metal screw base internal to the design. The threads molded in the socket are molded in sections, usually two, and only cover about 50% of the internal surface of the lamp base. The threaded sections are separated by cavities where there are no threads. One of these cavities has a thin, flat piece of metal (brass) running up the cavity, perpendicular to the threads. This metal bar contacts the threaded conducting base of the lamp by contacting the tips of the threads on the lamp as the bulb is screwed into the socket.
There is in the industry, especially due to price sensitivity, a considerable lack of uniformity in lamp base diameters and length. When a lamp is replaced with another lamp made by another manufacturer, it may not fit properly. If this replacement lamp is a little smaller in diameter then the lamp that it replaced, the quality of the electrical contact with this metal strip is poor at best, so lamp failure is immediate or inevitable. When a larger diameter lamp is screwed into the socket, it compresses this metal strip and, because this metal strip is made of brass, there is no return memory, so it stays compressed. This worsens the contact problems with other replacement lamps, plus the inability to get the lamp tight in the socket means that it will loosen more rapidly, failing even sooner.
The reduction of thread surface area reduces the ability of the socket to properly hold the lamp. Likewise, because the socket is plastic, and the C7/C9 lamps radiate so much heat, the sockets are constantly expanding and contracting. This expansion/contraction cycle is enhanced by the fact that most C7/C9 lamps hang downward, with the base directly in the path of the thermal given off by the bulb in the socket. This thermal coupled with normal temperature changes loosens the lamp via these expansion/contraction cycles. As the lamp loosens, the electrical contact fails first, then, especially outdoors, the loose lamp condition worsens from the wind and weather conditions swinging and moving the lamps, plus the expansion/contraction cycle. Everyone has attempted to replace a bad lamp at home only to find, when tightened, the lamp turns back on, that it was not burned out but only loose. This loosening happens in a full metal socket in doors, in an environment of constant temperaturexe2x80x94imagine what a plastic socket does when exposed to dramatic temperature changes, especially if the lamps are hanging upside down, which is the normal case regarding C7/C9 lamps due to their weight and simple fastening system.
Thirdly, still further cost cutting forces the manufacturer to use a very inexpensive method for connecting the metal conductors in the socket to the appropriate power lines. Electrical connection is achieved by forming the bottom of the socket terminals into spear pointed shapes which stick out of the bottom of the socket far enough to xe2x80x9cpiercexe2x80x9d the wire insulation and to make contact with the copper wire within, then a xe2x80x98snap inxe2x80x99 socket bottom is attached securing the wires to the socket. Nearly all of the C7/C9 sets use AWG 20, multi-strand wire. The plastic insulation is typically 2.4 mm in diameter, so the copper core being 20 AWG is very small in diameter compared to the plastic insulation. When the vinyl insulation is extruded onto the copper wire, the wire is somewhere near the middle of the vinyl coating if the machine is running properly. Just a slight change in wire feed of plastic extrusion, however, will cause the wire to be off-center in the insulation. Because the copper wire is never exactly in the middle of the insulation, the xe2x80x98piercing terminalsxe2x80x99 sometime miss or barely touch the copper wires. This causes the lamp to go on and off erratically or fail altogether, and is, in fact, one of the highest failure mechanisms to the C7/C9 sets. Also, partial or poor contact causes the wires to heat up and possibly burn during arcing inside the insulation.
In summary, there are problems with the C7/C9 sockets. These are:
1) C7/C9 sets are in parallel and are powered by 20 VAC, with electrical current limited by a slow blow, 5 7 amp fuse. An open socket is hazardous and could mean a severe, or lethal, electrical shock, or could be the energy source to start a fire in a bush or tree.
2) Some of the latest methods, in use today, use a 2 part center (AC Line) terminal that acts as a normally open switch, so when a lamp is missing, the center contact is suppose to open and disconnect from the power source. This terminal switch is made from a special copper that has a light spring action. If a replacement lamp is screwed into the base too far, it will compress this terminal beyond it""s ability to open, effectively defeating the normally open switch condition, thus leaving power applied to the center conductor when the lamp is removed, lost or missing. In addition to this failure mechanism, the center conductor""s copper spring contact requires very little force to close, so a small finger or limb can easily compress it enough to energize it.
3) A partially closed center terminal switch will arc. This arc can cause fire. This arcing will occur as the lamp loosens from expansion and contraction cycles.
4) This center terminal switch is subject to corrosion and poor contact and ultimately premature lamp failure due to condensation forming inside the socket as it cools. Likewise, this condition is exacerbated by rain or melted ice and snow, easily leaking into the socket due to poor sealing.
5) Threads in the C7/C9 sockets are formed into the plastic socket. These threads are partial, usually only 50% radius: 25% threads, 25% blank, 25% threads, 25% blank, running two thirds (⅔) to three quarters (xc2xe) of the depth of the socket. The reduction and interruption of threaded surface area reduces the ability of the socket to properly hold the lamp tightly, therefore allowing it to loosen quickly and fail to light or fall out.
6) The reduction and interruption of threaded surface area reduces the ability of the socket to properly seal to the bulb, and thereby to withstand inclement weather that can corrode internal components and accelerate premature lamp failure.
7) When a lamp is replaced with a general replacement lamp, there is a very good chance that the replacement lamp will not fit properly. If this replacement lamp is a little smaller in diameter, then the ability of the lamp to establish good contact with the AC neutral sidewall metal strip is poor at best, so lamp failure is immediate or inevitable. When a larger diameter lamp is screwed into the socket, it compresses this brass sidewall metal strip and because it is brass, there is no return memory, so it stays compressed. This worsens the contact problems with other replacement lamps, plus the inability to get the lamp tight in the socket means that it will loosen itself more rapidly, failing sooner then normal. Likewise, this can also cause internal arcing between the lamp base and the sidewall AC neutral power strip.
8) Due to the small gauge (20 AWG) wire used, the electrical contacting method using single piercing terminals is undependable because the copper wire is never exactly in the middle of the insulation. The xe2x80x98piercing terminalsxe2x80x99 will sometimes miss or barely touch the copper wire. This causes the lamp to go on and off erratically or fail altogether, and is in fact one of the highest failure mechanism to the C7/C9 sets. Partial or poor contact also causes the wires to heat up and burn from arcing inside the insulation as poor contacts worsen producing arcs caused from stress due to temperature changes and movement of the set during wind, ice and snow loads, and storms.
Thus there remains a need for an improved C7/C9 light socket.
Briefly recited, the present safety light socket overcomes the aforementioned problems by the use of several features in its design. Specifically, the C7/C9 Safety Socket has a nearly fully threaded interior to assure maximum seal between bulb and socket for the best possible weather resistance. It has a molded in, threaded, center terminal cover and lamp lock. This cover-and-lock component is molded in such a fashion that when the lamp in screwed into the socket, the cover-and-lock are pressed into a slot in the sidewall of the socket and held under constant pressure between the C7/C9 lamp base and the socket wall.
This design achieves several goals: by keeping constant pressure on the lamp base, and by having the threaded portion of the cover-and-lock formed with slightly oversized threads, the lamp becomes locked into the socket. Expansion/contraction cycles have no effect on the lamp as it is held at a constant pressure during expansion and the dissimilar thread sizes restrict turning ease so lamps are less likely to work themselves free.
When a lamp is removed, the terminal cover-and-lock expands to cover the center terminal, thereby helping to preventing accidental electrical contact. The larger threads on this part are rounder on the upper surface to help restrict penetration of foreign objects by having them slide off and veer off one or more of the threads to prevent full insertion, but still allow the threads to restrict fingers from entering the socket far enough to reach the center terminal. Due to the design, compression of this part requires considerable pressure, easily generated by the mechanical compression generated via the screw action of the lamp, but not easily generated by outside sliding/pushing motions from naturally occurring foreign objects such as bush or tree limbs.
The present invention uses double, xe2x80x98insulation displacementxe2x80x99 type electrical contacts. Insulation displacement type contacts cut partially through the insulation using an inverted xe2x80x9cVxe2x80x9d shape interior groove. This xe2x80x9cVxe2x80x9d shape groove xe2x80x98capturesxe2x80x99 the center conductor in the wire, whether it is off-center or not, and traps the copper conductors in the center of the xe2x80x9cVxe2x80x9d , tightly forming a xe2x80x9cGasxe2x80x9d fit. This contact keeps the electrical surfaces from oxidizing, thus preventing corrosion, arcing, and the resultant lamp failure, even if condensation forms internal to the socket. Each electrical contact has two of these insulation displacement contacts to assure the best possible connection.
Double, xe2x80x9cinsulation displacementxe2x80x9d type electrical contacts also grip and hold the electrical wires better then the piercing type contact. xe2x80x98Insulation displacementxe2x80x99 type electrical contacts maximize the pull strength of the socket so it is not easily moved along the conductors, as the insulation displacement xe2x80x9cVxe2x80x9d groove does not allow the wire insulation to be pulled through it, unlike the piercing type of contact which slices the wire insulation like a knife if the wires are pulled and the socket slides along the wires, thus exposing bare electrical wires and compromising safety.
These and other features and their advantages will be apparent to those skilled in the art of lighting systems from a careful reading of the Detailed Description of Preferred Embodiments, accompanied by the following Drawings: